1. Field of the Invention
The invention relates to silane-modified polyvinyl alcohols, to a novel process for preparing silane-modified polyvinyl alcohols and to their use.
2. Description of the Related Art
Polyvinyl alcohol and polyvinyl alcohol copolymers find various uses as binders, as protective colloids in emulsion and suspension polymerization, as cobinders in paper coatings and as an oxygen barrier in the packaging industry. The modification of polyvinyl alcohol with silane groups provides higher strengths and better attachment to mineral substances such as pigments, fillers and glass.
Silane-modified polyvinyl alcohols have been prepared to date by means of copolymerization of vinyl acetate and ethylenically unsaturated silane and subsequent hydrolysis, or by subsequent silylation of the vinyl alcohol units of polyvinyl alcohol with a silylating agent having at least two hydrolysable groups. EP 0076490 A1 describes a paper coating composition based on silanized polyvinyl alcohols which are obtainable by means of the two process variants mentioned. EP 1127706 A1 relates to inkjet recording materials which have a coating comprising silane-functional polyvinyl alcohol, the silanized polyvinyl alcohols used there likewise being obtainable by these two process variants mentioned. DE 3519575 C2, which relates to heat-sensitive recording materials having a protective layer, also mentions the alternatives of silylating the vinyl alcohol units or copolymerizing vinyl acetate with ethylenically unsaturated silane and subsequently hydrolysing as suitable preparation processes.
The silanized polyvinyl alcohols from EP 1380599 A1 and EP 1380600 are obtained by means of copolymerization of vinyl esters and ethylenically unsaturated silanes and subsequent hydrolysis of the silanized polyvinyl esters, and it is optionally possible to polymerize in the presence of mercapto compounds. EP 1080940 A2 describes three alternatives for the preparation of silanized polyvinyl alcohols: vinyl esters are copolymerized with olefinically unsaturated monomers having a silyl group and then hydrolysed. Vinyl ester is copolymerized with epoxy-functional comonomers, and the copolymer obtained thereby is reacted with a compound which contains both mercapto and silyl groups, and is then hydrolysed. Vinyl ester is copolymerized with mercapto and silyl groups in the presence of a compound and then hydrolysed. WO 2004/013190 A1 describes the preparation of silane-modified polyvinyl alcohols, vinyl esters being polymerized in the presence of silane-containing aldehydes, and the vinyl ester polymer obtained thereby subsequently being hydrolysed.
The processes mentioned are all relatively complicated, and, in addition, with increasing silane concentrations, gel can be formed during the synthesis and water-insoluble products can be formed owing to the side reactions of the hydroxyl groups with aldehyde.